31 December 2017

What is carnot cycle

The Carnot cycle is a thermodynamic cycle that can be invented by French Physicist Sadi Carnot in 1824.

What is the Carnot cycle?

In the Carnot cycle, two reversible isothermal processes and two reversible adiabatic processes are done in a heat engine.

Process of Carnot cycle :

The Carnot cycle consist of four processes following below :

  • A reversible isothermal gas expansion process. From the diagram shown below the ideal gas in the system absorbs some amount of heat from the heat source at a high temperature and then expands thus the work was done on surroundings. 
  • A reversible adiabatic gas expansion process. In this process, the system is thermally insulated. The gas expands continuously and does work on surrounding, which causes the system to cool to a lower temperature. 
  • A reversible isothermal gas compression process. In this process, work has done surrounding gas and cause a loss of heat.
  • A reversible adiabatic gas compression process. In this process, surrounding continue to do work to a gas, which causes the temperature to rise back to high temperature attain in the first process. 
Carnot cycle

The efficiency of the Carnot cycle :

It can be defined as the ratio of the energy output to the energy input. Here energy output is work done and energy input is heat addition.  

From the calculation,

Q1 = Heat addition = R T1 ln v2 / v1

Wnet = R ln v2 / v1 (T1 – T2 )

Ŋcycle = Wnet / Q1  =  T1 – T2 / T1

The large back work is a big drawback of this cycle. 

Assumption of the Carnot cycle :

  • No friction at all between the piston and cylinder and also other moving parts of the engine, thus there is no heat generated and lost due to friction.
  • There is no transfer of heat with the external atmosphere because the engine is completely insulated.
  • There is also no exchange of heat between various parts of the engine.